This invention relates generally to machine tools of the class having a horizontal, hollow main spindle through which a bar stock is fed by a bar stock feeding mechanism disposed coaxially to the rear of the main spindle and is gripped by a main spindle chuck disposed at the forward end of the main spindle to be machined by any of a plurality of tools held by a cutting tool holder.
More particularly, the invention relates to a device for intermittently feeding the bar stock for every cycle of machining in a machine tool of the above stated character, the main spindle being kept rotating continuously without stopping, and the bar stock also being rotated unitarily with the main spindle irrespective of the cross-section shape of the bar stock.
Heretofore, in a machine tool of the above stated character wherein a bar stock is successively fed in an intermittent manner, for every cycle of machining, from the rear of the main spindle into a machining zone, or area, the gripping parts of the feed chuck of the bar stock feeding device and of the main spindle are adapted and shaped to be suitable for gripping the outer surface of the bar stock, and the bar stock is caused to rotate unitarily with the main spindle. Two methods have been devised for newly feeding the bar stock by a specific length after every machining cycle. In the first method, the rotation of the main spindle is temporarily stopped. In the second method, the rotation of the main spindle is not stopped.
In the first method, the main spindle is braked after each machining cycle and stopped. Because this stopping is repeated with a high frequency, various difficulties arise. For example, the motor for driving the main spindle overheats, or play develops between parts to be tightened, of the main spindle assembly, whereby the precision of the main spindle is seriously affected, and this may become a cause of a breakdown or defective operation.
In the second method, there are problems particularly in the case of machining of bar stock of somewhat long length requiring center work or in the case of machining bar stock of long length by dividing the feeding thereof into the machining zone into two cycles thereby to attain some precision. More specifically, at the time of feeding the bar stock, the bar stock is released from the main spindle chuck by the loosening action thereof, but, in the interval during which it is supplied into the machining area, it is caused by centrifugal force to undergo head-shaking oscillation of the first overtone mode or of a higher order overtone mode. This oscillation will cause wear or damage of the gripping parts of the main spindle chuck and impart a considerable adverse effect on also the main spindle. In the case where the rotation speed of the main spindle is high, there will be a considerably high degree of danger. Furthermore, this oscillation in some cases imposes a limit to the feeding operation of the tools adjacent to the machining area or of the tool holder or head.
Still other problems have arisen in the prior art because of the manner of rotating the bar stock in the case where it is fed into the machining zone without stopping the main spindle. In this case, the rotations of the main spindle and of the bar stock at the time of its feeding have heretofore been unrelated to each other, and the bar stock is rotated merely by frictional contact thereof with the inner wall surface of the main spindle. This gives rise to a violent abrasion particularly between the bar stock and the chucks for gripping the bar stock, whereby there is severe wear and even breakage of the chucks. There is also a high possibility of damage to the inner wall surface of the main spindle.